This invention relates to the field of electro-optics and, more particularly, to optical detectors.
Optical detectors are of considerable importance in a number of areas of technology, particularly those detectors which operate in the infrared response range. The implementation of national missile defense and space surveillance programs, for example, has created a requirement for detection and imaging systems operating in the medium and long wavelength infrared spectral ranges. Furthermore, the performance criteria established for the parameters of resolution and field of view in these programs have led to the development of high density, large area arrays of long wavelength infrared (LWIR) and medium wavelength infrared (MWIR) detectors. Monolithic extrinsic silicon detector arrays appear to be ideally suited for such applications, since large scale integration (LSI) techniques, which have been extensively developed for silicon, can be combined with extrinsic silicon detector technology to fabricate a monolithic silicon focal plane. An additional constraint is imposed on the selection of the detector elements in such a system, however, since these systems frequently must operate in the presence of nuclear radiation. Nuclear radiation induced ionization pulses (spikes) at the detector output can introduce an added noise component which will reduce the capability of such a system to detect faint targets and will add to the burden of reliably interpreting the focal plane output. Although such nuclear radiation induced noise can be reduced by decreasing the thickness of the detectors, it has been demonstrated that extrinsic silicon detectors cannot be fabricated in the conventional photoconductor configuration with a thickness much less than approximately 100 microns (.mu.m) without sacrificing detector performance. Thin detector performance would be degraded because the high doping levels which would be required in a thinner detector to maintain a high quantum efficiency would result in unacceptable dark current levels due to "impurity banding" effects. See, e.g., P. R. Bratt, "Impurity Germanium and Silicon Infrared Detectors" in Semiconductors and Semimetals, Vol. 12, p. 89 (Academic Press 1977). This thickness limitation also adversely affects the level of optical cross talk which can occur between the detectors in an array. In addition, the thickness parameter prevents the use of epitaxial manufacturing approaches, although this consideration is not of primary importance. Furthermore, the performance of conventional extrinsic silicon detectors is degraded at low background levels by a number of response "anomalies" which make detector calibration difficult.
These limitations of conventional detectors in the infrared region are illustrative of difficulties experienced as well with nonsilicon detectors and detectors sensitive within other portions of the spectrum. Consequently, a need has developed in the art for an improved thin detector design.